1. Field of the Invention
This invention relates to an intake system for a multicylinder engine.
2. Description of the Prior Art
As disclosed in Japanese Unexamined Patent Publications Nos. 56(1981)-115818 and 57(1982)-51910, there has been proposed an intake system for a multicylinder engine in which discrete intake passage portions immediately connected to the respective cylinders are divided into two groups so that intake timings of the discrete intake passage portions (intake timings of the cylinders to which the discrete intake passage portions are respectively connected) in each group are not immediately successive, and the discrete intake passage portions in each group are concentrated to form a concentrated portion, branch intake passage portions extending upstream from the concentrated portions for the both groups and being integrated with each other to form a main intake passage portion. In the intake system, a supercharging effect can be obtained by virtue of periodical pressure vibration produced by the air column vibration system in the intake passage upstream of the concentrated portions (this supercharging effect will hereinbelow be referred to as "resonant supercharging effect"), and by concentrating the discrete intake passage portions whose intake timings are not immediately successive into each concentrated portion, intake air interference can be prevented.
When bypass air for adjusting the engine rpm during idling, blowby gas from the crankcase, fuel vapors trapped by the canister, or exhaust from the exhaust system is to be introduced into the intake passage in the intake system described above, such air or gas should be introduced into the concentrated portions or the portions between the respective concentrated portions and the junction of the branch intake passage portions extending upstream from the concentrated portions in order to evenly distribute the air or gas to all the cylinders. With respect to response to operation of the accelerator, it is preferred that a throttle valve be provided in each branch intake passage portion near the concentrated portion. In this case, said air or gas should be introduced into the concentrated portions or into the branch intake passage portions downstream of the throttle valves.
There arises a problem that when an introduction passage for introducing the air or gas into the intake passage is simply connected to each concentrated portion or branch intake passage portion, the concentrated portions are communicated with each other by way of the introduction passages and resonant supercharging effect can be adversely affected.
That is, the frequencies at which the pressure vibration in the concentrated portions is in tune with the vibration of the air column in the branch intake passage portions are in proportion to D/.sqroot.L, wherein L represents the average length of the branch intake passage portions between the respective concentrated portions and the junction of the branch intake passage portions and D represents the average diameters of the branch intake passage portions. Generally, an optimal torque increasing effect from the resonant supercharging can be obtained when the engine operates at low-to-medium speeds. When the concentrated portions are communicated by short introduction passages in an intake system which has been arranged so that the vibration of the air column in the portion of the intake passage upstream of the concentrated portions is in tune with the periodical pressure vibration in the concentrated portions in the low-to-medium engine speed range, the engine speed at which the two vibrations are attuned with each other becomes higher and, accordingly, the torque increasing effect of the resonant supercharging effect cannot be sufficiently enjoyed at the desired engine speed range.